Make systemd user configurable and create log directory

systemd/pgmon@.service

@@ -5,6 +5,8 @@ Before=pgmon.service
 After=network.target
 
 [Service]
+EnvironmentFile=/etc/pgmon/%i-service.conf
+User=${SERVICE_USER:-postgres}
 ExecStart=/usr/local/bin/pgmon /etc/pgmon/%i.yml
 ExecReload=kill -HUP $MAINPID
 Restart=on-failure